In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, specification of a device-to-device (D2D) proximity service has been promoted as a new feature after Release 12 (see Non Patent Document 1).
The D2D proximity service (D2D ProSe) is a service for enabling direct device-to-device communication within a synchronization cluster including a plurality of pieces of user terminals which are synchronized with one another. The D2D proximity service includes discovery procedures (Discovery) of discovering a proximity terminals and D2D communication (Communication) that is direct device-to-device communication.
As an application for the D2D proximity service (hereinafter, referred to as a D2D application), a public safety application, a vehicle-to-vehicle communication application, and the like are assumed, besides a phone application.